Approximately half of the metastatic melanoma patients develop brain metastases. Despite recent advances in therapy for metastatic melanoma, treating patients with brain metastases remains challenging. These patients are typically excluded from clinical trials because historically, melanoma brain metastases (MBMs) were difficult to control and usually portended a poor prognosis. Systemic therapies for patients with MBMs are understudied and preclinical development of new effective therapies are limited by the lack of knowledge regarding mechanisms involved in brain metastasis, coupled with the lack of studies addressing drug passage across the blood brain barrier (BBB). This emphasizes the immediate need for molecular studies of MBMs with the goal of developing new drug targets and effective and specific systemic therapies. In our preliminary studies we conducted gene expression profiling of tumors of patients who developed early brain metastases and compared them to patients who did not; PLEKHA5, a gene known to regulate normal brain development, and CEACAM1, an immune checkpoint molecule, were among the most differentially expressed genes. In situ assessment of protein levels in a non-overlapping set of patients showed that high levels of both molecules are significantly associated with propensity for brain metastasis, while the risk of developing early MBMs is highest when tumors have elevated Ceacam1 levels and low density of tumor infiltrating lymphocytes. Moreover, patients who presented with MBM as their first distant site of disease had highly elevated levels of both biomarkers. Knock-down of PLEKHA5 decreased in vitro and in vivo proliferation of cerebrotropic cells and inhibited their BBB transmigration and invasion. Further analysis showed a possible interplay with the PI3K pathway. Here we propose to 1) further study PLEKHA5 and CEACAM1 as possible mediators of MBMs utilizing somatic cell genetic approaches along with in vitro cell based assays and in vivo experimental mouse models recently developed, and to 2) concurrently investigate the role of CEACAM1?directed immune modulation in brain metastasis. The proposed studies are of high clinical relevance: first, the PI3K pathway has been shown to be important and druggable in MBMs with clinically available compounds that cross the blood-brain-barrier and second, a newly developed immune check point inhibitor, directed against CEACAM1, is being investigated in clinical trials in solid tumors including melanoma. Our studies will provide meaningful information regarding the roles of PLEKHA5, CEACAM1 and their combined roles in brain metastasis. If successful, these studies will form the basis for future development of PLEKHA5/PI3K and/or CEACAM1- targeting therapies for treatment of melanomas prone to metastasize to the brain. Moreover, they will provide insight into biomarker-based patient selection for these trials, resulting in an improved therapeutic window.